This invention relates to the field of thin film materials and more particularly the invention relates to methods and apparatus for measuring or sensing magnetostriction of thin film materials.
Ferromagnetic and nonferromagnetic thin film materials are widely used in transducers and disks in magnetic disk drives, as well as, the semiconductor industry. The films are deposited by various processes including CVD, electroplating and sputtering.
The magnetostriction of magnetic materials such as NiFe can be correlated with the composition of the materials. Thus, it known to be useful to measure the magnetostriction of the thin films deposited in the process of making magnetic heads as a way of monitoring the quality of the process. The current way that this is done, is to remove the wafers on which the films have been deposited from the process and perform the measurements in a laboratory. This obviously involves delay and considerable expense. Thus, there is a need for a way to conveniently test the magnetostriction of materials on a wafer.
Shinji Saitou (U.S. Pat. No. 5,589,770) describes a mechanical sensor for the measurement of stress or distortion using magnetostrictive distortion in a film by layering a magneto-resistive film on top of the magnetostrictive film with an intervening thin electrically insulating film. Saitou""s mechanical sensor comprises an initial layer of an amorphous ferromagnetic material having magnetostrictive properties preferably comprising at least one alloy selected from the group comprising Fexe2x80x94Crxe2x80x94Sixe2x80x94B-based, Fexe2x80x94Nbxe2x80x94Sixe2x80x94B-based, Fexe2x80x94Vxe2x80x94Sixe2x80x94B-based, Fexe2x80x94Coxe2x80x94Sixe2x80x94B-based, Fexe2x80x94Wxe2x80x94Sixe2x80x94B-based, Fexe2x80x94Nixe2x80x94Crxe2x80x94Sixe2x80x94B-based, Fexe2x80x94Nixe2x80x94Nbxe2x80x94B-based, and Fexe2x80x94Nixe2x80x94Moxe2x80x94B-based alloy. Saitou stated that it is preferable in the above-mentioned configuration that the amorphous magnetostrictive alloy comprise Fe75Cr4Si12.5B8.5 and that the electrically insulating layer is preferably SiO2. The second ferromagnetic layer must have a significant magneto-resistance effect and is preferably a NiFe alloy. A magnetic field generating means excites both of the ferromagnetic layers, and a change of magnetic flux density passing through the ferromagnetic layers resulting from a change of magnetic properties due to stress is detected as a change of resistance value arising from the magneto-resistance effect. Thus, Saitou""s mechanical sensor uses magnetoresistance and magnetostriction to measure strain.
Given a thin film magnetic material deposited on a substrate such as a silicon wafer, a sensor for the magnetostriction of the magnetic film can be constructed by depositing a thin film nonferromagnetic electrically insulating layer followed by a thin film layer of a sensor material which is piezoresistive and nonferromagnetic. The insulating and the piezoresistive film can be etched into an appropriate pattern and orientation to provide sensitivity to strain vectors in the magnetic film as desired. The magnetostrictive strain in the magnetic film induced by a known magnetic field induces a corresponding strain in the piezoresistive film which can be measured as a change in the electrical resistance which can be detected by external probes or other measuring means. The measurement of the magnetostriction can be performed as a part of the manufacturing process and does not require that the wafers be removed to a laboratory. It is preferable that the insulating and sensor layers be thinner than the magnetic layer to maximize the transmission of the strain. Duplicate piezoresistive films can be deposited over insulated areas where there is no underlying magnetostrictive film to provide a value for a differential measurement.
The preferred embodiment described below uses the sensor as a part of the process of manufacturing the wafers from which a plurality of magnetic heads will be cut for use in disk or tape drives. The sensors can be built in otherwise unused locations on the wafer, for example, in the saw kerf areas. The sensors may be built and used when the wafer is in an intermediate point in the build process and/or after the heads on the wafer are essentially complete.